1. Field of the Invention
The present invention relates to methods for manufacturing semiconductor optical modulators and to semiconductor optical modulators.
2. Description of the Related Art
T. H. Stievater et. al., “Enhanced Electro-Optic Phase Shifts in Suspended Waveguides”, OPTICS EXPRESS, 18 Jan. 2010, Vol. 18, No. 2, pp. 885-892 (Ref. 1) discusses a method for manufacturing a semiconductor optical waveguide for a semiconductor optical modulator. In this method for manufacturing an optical waveguide, a p-type AlInAs semiconductor layer is formed on a p-type semiconductor substrate. Then, stacked layers including a core layer are formed on the p-type semiconductor layer. The core layer has a multi-quantum well (MQW) structure. The p-type semiconductor layer and the stacked layers form an optical waveguide structure. Holes are then formed in side surfaces of the optical waveguide structure, and the p-type semiconductor layer is wet-etched through the holes in a direction from the side surfaces along the waveguiding direction. In this way, a gap extending in the waveguiding direction is formed in the p-type semiconductor layer. The gap reduces the amount of p-type semiconductor layer in the optical waveguide structure. In general, a p-type semiconductor layer has a larger optical absorption loss than that of an n-type semiconductor layer. Therefore, this, as discussed in Ref. 1, reduces an optical absorption loss in the optical waveguide structure.
In the method for manufacturing a semiconductor optical waveguide according to Ref. 1, as described above, the p-type semiconductor layer is wet-etched in a direction from the side surfaces along the waveguiding direction to reduce the amount of p-type semiconductor layer in the optical waveguide structure. In the semiconductor optical waveguide according to Ref. 1, the amount of p-type semiconductor layer is reduced to reduce an optical loss in the optical waveguide structure. When the p-type semiconductor layer is etched, additionally, it is necessary to control the etching depth and to maintain a uniform depth. One approach to controlling the etching depth is to provide an etch stop layer between the p-type semiconductor substrate and the p-type semiconductor layer. The amount of etching can be controlled in the depth direction by providing the etch stop layer. The control of the amount of etching with an etch stop layer, however, is not suitable for controlling the gap width (the width perpendicular to the waveguiding direction). Instead, the gap width is controlled depending on parameters such as the etching rate and etching time of wet etching. The gap width needs to be uniform with micron-level accuracy over the whole length of the optical waveguide, namely, several millimeters.
For the method for manufacturing a semiconductor optical waveguide by wet etching, however, it is difficult to uniformly remove the p-type semiconductor layer from the optical waveguide structure in the waveguiding direction because the gap width is controlled depending on the parameters such as the etching rate and etching time of wet etching. For the method for manufacturing a semiconductor optical waveguide according to Ref. 1, specifically, it is difficult to form a gap with a uniform shape (e.g., a uniform cross-section perpendicular to the waveguiding direction). As a result, the amount of p-type semiconductor layer removed from the optical waveguide structure varies in the waveguiding direction. This variation contributes to scattering of light propagating through the optical waveguide structure and also to increasing an optical loss due to the scattering. It is therefore difficult to sufficiently reduce an optical propagation loss in a semiconductor optical modulator including a semiconductor optical waveguide manufactured by the method as above.